The search for a maximum of the D-π-A paradigm for second order nonlinear optical molecular materials

Push-pull pi-conjugated molecules are one of the paradigms of second order nonlinear optical (NLO) materials and have been extensively explored. However, high-performance second order NLO materials with an optimum electron donor (D), pi-bridge (pi) and acceptor (A) under this paradigm are still the most sought-after. In the present work, D-pi-A molecules with optimal D, pi and A combination for strong second order NLO properties are proposed based on molecular orbital theories. The optimal D-pi-A push-pull molecule achieves an unprecedentedly strong NLO response under the D-pi-A paradigm, i.e., the static first hyperpolarizability reaches -453.92 x 10-30 esu per heavy atom using azulene as part of the pi-bridge and acceptor to synergistically reinforce the strength of the acceptor. The protocols of D-pi-A NLO molecule design through frontier molecular orbital matching of D, pi and A with optimal combination of electron donating and accepting strengths shed light on future molecular NLO materials exploration. The simulated two-dimensional second order spectra provide useful information (e.g., sum frequency generation) on the applications of those D-pi-A push-pull molecules in nonlinear optics. The proposed D-pi-A molecule design protocol through frontier molecular orbital matching based on molecular orbital theory provides an effective means to design novel high-performance nonlinear and optoelectronic molecular functional materials.

Automated summary

This study proposes a D-π-A molecule design protocol based on molecular orbital theory, which achieves the optimal combination of D, π, and A for strong second order nonlinear optical response. By simulating two-dimensional second order spectra, useful information is provided for the application of these D-π-A push-pull molecules in nonlinear optics, offering new insights for the development of future molecular nonlinear optical materials.